The present invention relates to a system for controlling active suspensions of vehicles.
Active suspension systems of various forms have been developed and publicly disclosed as in Japanese Pat. Appln. Laid-Open Publn. No. 62-139709. A typical example of a known active suspension system comprises the following essential components. Individual suspensions are provided for each wheel for supporting the vehicle by fluid pressure. Charge and discharge of the fluid into and out of each suspension is controlled independently by the operation of a respective control valve. The operation of each control valve is controlled by opening and closing control signals generated by a controller which, responsive to information such as vertical acceleration of the vehicle mass above suspension units and extension and contraction displacement strokes of the suspensions, calculates command .quantity of charge or discharge of fluid for each suspension unit. Thus the charge and discharge of the fluid into and out of each suspension is controlled.
Applicant has previously developed an active suspension system of the following description and has filed a U.S. patent application therefor (U.S. application Ser. No. 410,834). In the active suspension system as described above, sensors are used to detect accelerations in the longitudinal and lateral directions of the vehicle. The above described controller preestimates variations of the vehicle attitude (pitching and rolling) accompanying acceleration, deceleration or turning of the vehicle in response to the information from the sensors. Then, the controller computes command quantity of the charge and discharge of the fluid for maintaining the vehicle attitude in a desirable state, thus generating and transmitting signals for opening and closing the control valves.
When a vehicle is traveling on an even road surface, the higher the vehicle speed is, the larger is the longitudinal acceleration component due to vibrational motions of the vehicle.
In an active suspension system having a controller for controlling the vehicle attitude on the basis of detected value of the vehicle longitudinal acceleration (hereinafter referred to simply as longitudinal G) as described above, the longitudinal acceleration component arising from the vibrational motions of the vehicle is added as microvibration, i.e., noise, to the detection signal of the longitudinal acceleration sensor for detecting the longitudinal G. For this reason, if the vehicle attitude is controlled on the basis of the resulting detection signal, unnecessary control is performed so that not only does waste of energy occur, but there arises a possibility of instability of the vehicle attitude.
In order to eliminate the noise of the detection signal, the use of a low pass filter may be one solution. However, when a low-pass filter is used, the rise of the signal of the longitudinal G sensor is delayed at sudden braking. Consequently it becomes impossible to achieve the original object of controlling the vehicle attitude precisely and positively without delay.